Is Jiang entirely fictional or rooted in a historical character? I remember seeing a chinese history show where the narrator talked about how Zhou defeated Shang in only 3 months. That's faster than the Americans taking over Iraq.
Actually it take quite a number of years for Zhou to completely annihilate the Shang. And the story is quite long with Jiang actually helping the Shang in the beginning.
However the feng shen ban (or Feng Shen Yan Yi) is a story, more like a legend of the beginning of the Taoist gods. It is a collection of how many deities came about.
I do believe that there might really be a person call Jiang Zi Ya, but I do not believe all those magical thingies. It is just not scientific. Actually when you look at it, it was simply a war between two nations - Shang and Zhou, and in the beginning Zhou is a small vessal state of Shang, and they rebel and won the war ultimately.
To make thing more interesting, both the generals from Shang and Zhou are make deities and they live happily ever after, which actually reflected ancient Chinese believes and dreams - to attain immortality.